The present invention relates to vibration actuators. More particularly, the present invention relates to vibration actuators which generate a driving force through elliptic motion in a surface of an elastic member.
Elliptic motion in a vibration actuator results from excitation of an elastic member by an electro-mechanical converting element. Relative motion is then produced between the elastic member and a relative moving member which is in compressive contact with the elastic member.
Conventional vibration actuators cause multiple vibrations of an elastic member by impressing alternating electric currents onto electro-mechanical converting elements which are attached to the elastic member. The construction and load characteristics of a conventional vibration actuator are described in "Piezoelectric Linear Motor Intended for Optical Pickup Movement" (Mr. Yoshiro Tomikawa et al., 5th Electromagnetic Force Related Dynamics Symposium, Collected Papers, 1993, pp. 393-398).
FIGS. 9A-9D illustrate a conventional vibration actuator 1. FIG. 9A is a top view, FIG. 9B is a front view, FIG. 9C is a right side view, and FIG. 9D is a bottom view. Vibration actuator 1 is in the form of a rectangular parallelepiped, however a rectangular plate form is particularly illustrated in the figures. Vibration actuator 1 includes an elastic member 2 which includes driving force output units 2a, 2b formed as projections on a first flat surface of elastic member 2. A relative moving member 3 is in compressive contact with elastic member 2 via driving force output units 2a, 2b and a compression mechanism (not shown). Driving force output units 2a, 2b are respectively formed at two locations which are close to antinode positions of a fourth order bending vibration generated in elastic member 2. A number of materials may be used for the construction of elastic member 2, including stainless steel, aluminum alloy and like metallic materials, or plastic materials.
Piezoelectric elements 4a, 4b, 4p, 4p' are affixed to a second flat surface of elastic member 2 opposite to the first flat surface. The piezoelectric elements are electro-mechanical converting elements in the form of rectangular thin sheets.
Piezoelectric elements 4a and 4b are drive elements for generating bending vibrations and longitudinal vibrations in elastic member 2. Piezoelectric elements 4p, 4p' are mechano-electric converting elements which detect a vibrational state generated in elastic member 2. Lead wires (not shown) are soldered to piezoelectric elements 4p, 4p', and are similarly connected to a control circuit (not shown).
When alternating voltages, which are drive signals from a drive voltage generating device (not shown) are impressed onto piezoelectric elements 4a, 4b, first order longitudinal vibrations and fourth order bending vibrations are generated in elastic member 2. As a result, when relative moving member 3 is in compressive contact with elastic member 2, via driving force output units 2a, 2b, relative motion is generated with respect to elastic member 2. This relative motion is then used as an output force for driving an exterior unit.
In vibration actuator 1, respective fixed frequencies of a first order longitudinal vibration and a fourth order bending vibration are set such that they become very close or are set to the same value. Accordingly, by impressing alternating voltages having two close, fixed frequencies, onto piezoelectric elements 4a, 4b, respectively, a first order longitudinal vibration and a fourth order bending vibration are harmonically generated.
End surfaces of driving force output units 2a, 2b are formed as projections and respectively support sliding members 5a, 5b. Sliding members 5a, 5b are affixed onto the entire end surfaces of driving force output units 2a, 2b in order to reduce a sliding resistance with relative moving member 3. Conventional sliding members 5a, 5b are uniformly formed by affixing plastic, metal plating, or a melted coating of inorganic materials and the like. FIG. 9C particularly illustrates sliding members 5a, 5b with a portion broken away.
However, in vibration actuator 1 illustrated in FIGS. 9A-9D, even if careful attention is paid to the optimal selection and affixing of sliding members 5a, 5b, slip arises between the sliding members 5a, 5b and relative moving member 3 during driving. Because of this, sliding noise is generated by the sliding members. This presents a problem because silent operation is an important characterizing feature of vibration actuators. The generation of this type of sliding noise also limits an optimum range of vibration actuator 1 and is one of the problems that needs to be solved.
Furthermore, the energy of the longitudinal vibrations and bending vibrations generated in elastic member 2 is transmitted to relative moving member 3 via sliding members 5a, 5b. Thus, generation of this type of slip reduces drive efficiency of vibration actuator 1.